TimW wrote:
On 05/09/2020 10:31, newshound wrote:


Difficult to comment without knowing a bit more about the materials,
construction, and any constraints. I'd have thought that with
sufficient screws you would have enough structural strength.

I'd think about using polycarbonate rather than perspex.


Yeah! the 3mm acrylic is rubbish. I broke two of the three pieces just
drilling and screwing it!

TW

For plastic work, I like these. Countersink bits.

https://images.homedepot-static.com/...03-64_1000.jpg

I'm not using the "chewy" part, the collar that does
the countersinking. I've pushed that away from the work
and closer to the drill chuck.

To drill a hole in plastic, start with one of the
smaller bits you've got. Say a 1/16th inch regular HSS.
Drills tend to skate on plastic, and a smaller drill bit
might stay put. That's your pilot hole, for anti-skate.

Then, using your first countersink bit, "ream" out the hole.
Stop before you get to the desired diameter. If you try
to advance the bit too fast, it heats up and fills the
cutting part with molten plastic.

I made LED lights for the bicycle, and the light was a
light array. To mount the array, I used perspex sheet and
drilled 48 holes in it (a bag of LEDs...). The holes are
perfect and didn't crack the plastic. But it did take
a while to drill the 12 x 4 matrix.

Regular drill bits, they catch in the work and crack it.
That's how I ruined the first one.

And no matter what kind of drill bit you use, they tend
to clog with molten plastic and then need cleaning. You're
constantly cleaning the blasted things.

I don't have a whole set of countersink bits, just two
bought individually, so I lack a nice selection for easy work.
I can't go all the way up through the drill index sizes that
way, because my set isn't complete.

Once you're close to the correct size, you'll still need
a regular drill bit to make the hole the same diameter
all the way through. Which may or may not be important,
depending on what you're doing. At the larger bit sizes,
the regular bit can still catch in the work and crack it.
Even with a very nice pilot hole to guide it, there's
still a risk.

And polycarbonate ? Try drilling a CD first, and maybe that'll save
you buying a sheet of the stuff somewhere. See if you like it.

If you drill a deep enough hole in plastic, it will
"pull off to the side". If tapping a hole and attempting
to place a bolt in the freshly tapped hole, you may notice
"****, the bolt is binding". Attempts to run the tap through
again, still find the bolt binding. This happens because your
hole actually zings off square. And the bolt going into
the threaded hole, the hole is crooked so the bolt binds.

The end result is, for a tapped hole, tap to a depth of
one inch, and use half inch bolts (in other words, use only
half of the threaded portion, the half that still runs true).

I use #2 SS bolts and buy perspex thick enough to use them. I
can make screw-together boxes out of perspex that way.

The machinist made beautiful slabs for gluing, and his
work is so nice, you could make an aquarium without screws.
Just using subtractive glue and his nicely milled edges
was enough. That's how my low pressure laser cavity was
made (ran at 1 torr). My technique here is bad enough,
that would never work. That's why I just screw them together
and they're not watertight.

Paul